Lubrication of contact surfaces

ABSTRACT

Surfaces of electrical contact members, which move relatively over each other, are lubricated with discrete particles of graphite firmly bonded to at least one of the surfaces, the particles having their basal planes extending substantially parallel to the plane of the surface, and the particles further having undergone shear so that the platelets, which comprise each particle, form an imbrecated structure on the surface. Only a minor portion of the surface is covered with the particles so that the ability of the surface to pass an electric current is not degraded. Particles may be applied by spraying the surface with a gaseous stream having entrained particles therein.

United States Patent [72] Inventors Rayond James Geckle;

Larry John Wilt, both of Harrisburg, Pa. [21] Appl. No. 845,478 [22] Filed July 28, 1969 [45] Patented Nov. 16, 1971 [73] Assignee AMP Incorporated llarrisburgb, Pa.

[54] LUBRICATION 0F CONTACT SURFACES 1,035,730 German Auslegeschrift PFLAUM" Aug. 1958. 1 l7-- 226 Primary Examiner-William L. Jarvis Attorneys-Curtis, Morris and Safford, Marshall M.

l-lolcombe, William l-lintze, William J. Keating, Frederick W. Raring, John R. Hopkins, Adrian J LaRue and Jay L. Seitchik ABSTRACT: Surfaces of electrical contact members, which move relatively over each other, are lubricated with discrete particles of graphite firmly bonded to at least one of the surfaces, the particles having their basal planes extending substantially parallel to the plane of the surface, and the particles further having undergone shear so that the platelets, which comprise each particle, form an imbrecated structure on the surface. Only a minor portion of the surface is covered'with the particles so that the ability of the surface to pass an electric current is not degraded. Particles may be applied by spraying the surface with a gaseous stream having entrained particles therein.

PATENTEDunv 16 IE'II 3, 520, 39

sum 2 0F 2 LUBRICATION OF CONTACT SURFACES BACKGROUND OF THE INVENTION This invention relates to lubrication of surfaces which move relatively over each other and particularly to the surfaces of electrical contact members.

A wide variety of electrical devices having contact surfaces, such as switches or disengageable electrical connecting devices, are used under greatly varying conditions as regards voltages and currents. Under many circumstances, satisfactory electrical contact can be achieved at the interface of the contact members without special preparation or maintenance of their surfaces; for example, a heavyduty electrical switch intended for high voltage, high current applications can be made of unplated copper and the switch parts can be designed such that extremely high contact pressure is imposed when the contact surfaces are engaged with each other. Under such circumstances, the contact surfaces will clean themselves as they move over each other during opening and closing of the switch. Furthermore, because of the high voltages involved, minor amounts of oxide or other foreign matter on the contact surfaces will not interfere with the function of the switch. If desired, the contact surfaces can be manually cleaned and they may be lubricated to facilitate opening and closing of the switch although such lubrication is not needed to prolong switch life or for any other reason.

Other types of contact devices having sliding contact surfaces are used for low-voltage low-current applications, such as the contact terminals used in printed circuit board connectors and the pin and socket type contact terminals used in multicontact connectors. Contact devices of these types may be used in relatively complex electronic equipment and often must be of small size because of the vast numbers of contacts required in a given volume. These contacts frequently carry very low-voltage low-current signals and must possess an ex tremely high degree of reliability for the reason that the functioning of the equipment in which they are installed depends upon the operation of large numbers of the terminals.

It -can be appreciated from the foregoing that the maintenance of good electrical contact in small connecting devices used in low-voltage, low-current applications is of considerably more importance than the maintenance of electrical contact in heavy duty switches intended for high-voltage use. As a practical matter, the surfaces of the contact terminals used in printed circuit board connectors or the like cannot be periodically cleaned because of the vast numbers of terminals involved. The contact surfaces must remain clean, however, because of the fact that the low voltages involved are not sufficient to break down even an extremely thin oxide film on the surfaces.

The term dry circuit conditions" is commonly used in the electronics industry to specify the performance requirements of contact terminals designed for extremely low-voltage, lowcurrent applications where a high degree of reliability is necessary and where only a limited millivolt drop can be tolerated at the electrical interface. The achievement of electrical connectors capable of operating under dry circuit conditions has provided a continuing challenge to the ingenuity of the design engineer, who is charged with the responsibility of producing an extremely small terminal having a high degree of physical reliability, and the metallurgist, who must provide surfaces on the contacts which will meet the performance requirements of the industry as regards conducting ability, wear resistance and resistance to the formation of oxides.

The high standards of performance required of first quality small electrical contact terminals are often achieved by plating the surfaces of the contacts with gold or other noble metal. Gold is a preferred contact material because of its extremely high resistance to corrosion and its excellent electrical conductivity. From a purely electrical standpoint, pure (unalloyed) gold is an ideal plating for electrical contact terminals because of the fact that pure gold has better electrical conductivity than any possible gold alloy containing a minor amount of another metal and because of the additional fact that pure gold is relatively ductile so that when two contact terminals whose surfaces are plated with pure gold are engaged with each other, the surface platings will plastically deform as the parts are engaged and conform to each other to provide an extensive interfacial contact area for the passage of the electric current. The ductility of gold is, however, a decided disadvantage for disengageable electrical contact members from a wear standpoint. Ductile or soft gold is rapidly worn away from a sliding contact surface so that the base metal or underplating is exposed, which condition leads to the formation of oxide films and degradation of the current carrying ability of the terminal. Such rapid wear and degradation is intolerable in multicontact connectors which must be repeatedly engaged with, and disengaged from each other. In some instances, it is required that disengageable connectors be able to withstand 1,000 cycles of engagement and disengagement without significant degradation of the contact surfaces of the terminals in the connector.

It is present practice in the electronics industry to codeposit at least one additional metal with gold when contact tenninals are plated in order to harden the gold and thus improve the wear characteristics of the plating. The metals nickel, cobalt, silver, and iron are commonly used for this purpose. However the precise alloy chosen, must represent a compromise for whenever an additional metal is alloyed with the gold, the electrical conductivity of the gold is decreased to a substantial degree. Furthennore, the harder surface layer produced by codeposition of gold with another metal will not plastically deform as readily as a soft pure gold plating, and the quality of the contact interface suffers as a result.

It should also be mentioned that, for economic reasons, the thickness of gold platings on contact terminals must be kept to the absolute minimum which is consistent with fulfillment of the performance requirements of the terminal. Platings as thin as thirty millionths of an inch are commonly used on high quality contact terminals and the cost of a plating even this thin results in a substantial increase in the overall cost of the terminal.

It follows from the foregoing discussion that lubrication of the contact surfaces of small electrical contact terminals, particularly when used under dry circuit conditions, would be highly desirable for several reasons, In the first place, the wear of the surfaces during engagement and disengagement of the connectors would be substantially reduced so that a plating of a given hardness and thickness would withstand many more engagement-disengagement cycles with suitable lubrication than it would be able to withstand without such lubrication. Furthermore, if the surfaces are lubricated, the possibility exists of reducing or eliminating the alloy content in the gold plating so that pure gold having the best possible electrical characteristics could be substituted for the gold alloys currently in use.

The use of conventional lubricants for gold contact surfaces in electrical connectors is not however completely practical. If for example, a liquid lubricant, a hydrocarbon oil or a grease, is used, the surface will attract small particles of dust which might eventually counteract any benefit conferred by the lubricant and result in a failure of the tenninals involved. Furthermore, most liquid lubricants are incapable of withstanding elevated temperatures even for a short time, and the performance requirements for high quality electrical connectors usually include an elevated temperature test which could not be passed by most liquid lubricants. The life of a conventional oil or other liquid lubricant would also be quite limited since the terminals having a thin film of lubricant thereon would be continually exposed to the atmosphere so that evaporation and/or degradation of the lubricant would be expected. Finally, it would be difficult to determine if a conventional lubricant had been applied to a contact terminal at the time of manufacture and, after use of the terminal for some time, whether the film was still present and capable of carrying out its lubricating function. For these and other reasons, most small contact terminals, including those used for dry circuit applications, are not lubricated in any manner and the performance expectations are predicted on the assumption that the surfaces will not wear to a degree sufficient to cause degradation after a predetermined number of insertion and extraction cycles.

We have discovered that sliding electrical contact surfaces, can be effectively lubricated if minor portions of the surfaces of the terminals are covered with extremely small graphite particles which are bonded to the surface in a preferred crystallographic orientation such that the basal planes of the particles extend substantially parallel to the plane of the contact surface.

The particles of graphite are applied to the contact surface in a manner such that each particle is under a shear stress at the instant of application. The particles therefore undergo basal shear at the instant of application and each particle, after arrival at the surface, comprises at least two graphite platelets, one of which overlaps the other. It has been found that if about to percent of the area of the contact surface is covered with such particles, optimum lubrication of the contact surfaces will be achieved and that the wear or galling of the surfaces on engagement will be substantially reduced. As a consequence, a plating of a given thickness will be rendered capable of withstanding an increased number of wear cycles (that is engagements with a complimentary surface) prior to breakdown of the surface. This means that for a given number of cycles, a thinner plating can be used or a softer gold can be substituted for a hard alloyed gold to provide improved electrical characteristics. The particles adhere to the contact surface with a high degree of tenacity and appear to be bonded to the surface. The particles cannot be removed by ultrasonic cleaning with conventional solvents or degreases nor will they be disturbed by any physical means, such as handling, which does not result in removal of the gold plating itself. The particles can be barely seen with the aid of an optical microscope and can be studied in detail only by the methods of electron microscopy. 7

While the foregoing discussion is directed primarily to the use of the invention on gold-plated surfaces, it is to be understood that the principles of the invention can be used with a variety of types of contact surfaces where a wear problem or an insertion force problem exists. Thus, a contact surface can be treated in accordance with the invention to reduce the force required to engage the contact surface with a complimentary surface. As will be pointed out below, the principles of the invention can also be applied to reduce wear on sliding switches having nickel and beryllium copper contact surfaces.

The invention is described in further detail below and illustrated by the accompanying drawing in which:

FIG. 1 is a schematic representation of the manner in which graphite particles are applied to a contact surface.

FIG. 2 is an electron photomicrograph of an individual particle on a surface.

FIG. 3 is a perspective view of an apparatus for applying graphite to a contact surface in accordance with the invention.

FIG. 3A is a sectional view of the chamber of apparatus of FIG. 3.

FIG. 4 is a schematic sectional side view of two contact surfaces illustrating the manner in which the surfaces are lubricated by graphite particles.

In the description which follows, the physical structure of a graphitized surface in accordance with the invention is described in detail to the extent possible, the detail being somewhat limited in view of the extremely small size of the graphite particles. Additionally, preferred methods of graphitizing the surfaces of the contact members are described to permit those skilled in the art to duplicate the disclosed processes. Applicants also present below a discussion of the invention including hypotheses relating to the manner in which the particles are deposited on, and bonded to, the surface and the role the particles play in lubricating the contact surfaces. It is to be understood that applicants do not intend to be limited to the particular hypotheses presented and that the discussion of the theoretical aspects of the invention are presented in the interests of completeness, as a contribution to the literature on graphite, and in order that future workers in the field may comment upon, and extend, applicants findings.

A lubricating effect in accordance with the invention can advantageously be provided on the contact surface 10 of a conventional printed circuit board contact terminal 8 (FIG. 3) which is adapted to be crimped onto a wire. Contacts of the type shown at 8 are ordinarily mounted in insulating housings which are adapted to be engaged with printed circuit boards so that the individual contact terminals will engage the conductors of the board. Terminals of this type are frequently plated with thin platings of gold and the connectors must be repeatedly engaged with and disengaged from a printed circuit board so that a lubrication affect, which would prolong the life of the plating and/or permit use of a softer or thinner gold plating, would be desirable.

When the surface 10 is graphitized in accordance with the invention, about 10 to 20 percent of its area will have bonded thereto graphite particles of the general type shown in FIG. 2, this Figure being a 30,000X electron photomicrograph of an individual particle. The particle 4 of FIG. 2 has an average transverse dimension (extending substantially parallel to the plane of the surface) of about 6 microns and comprises two or more layers 5, each of which comprises a graphite platelet, one of which overlaps the other. As will be explained below, the two platelets shown in FIG. 2 were originally in alignment with each other but at the time the particle was deposited on the surface, shear occurred between two sections of the particle so that one section was displaced laterally with respect to the other section to yield the structure of FIG. 2. It should be explained that the term shear is used above in the sense of its definition to the physical metallurgist, namely a type of deformation in which parallel planes in metalcrystals slide so as to retain their parallel relation to one another, resulting in block movement" (Metals Handbook, I948 Edition, American Society for Metals, Cleveland, Ohio).

Exhaustive studies of particles of the type shown in FIG. 2 indicate that these particles are single crystallites, that is, crystal lographically homogenous domains of graphite, having their basal planes extending substantially parallel to the plane of the contact surface. This observation is consonant with experimentally determined values of the elastic compliance parameters of graphite single crystals. It has been found that for basal shear. S 4350 X 10- cmfl/dyne. whereas for prismatic shear on {E00} and {1150} planes, s .,=11.1 10- cm /dyne and S, =-0.4-6 X 10'' cm /dyne. The above values of elastic compliance are to be interpreted in the sense that the greater the value of the S parameter, the easier is shear on the relevant crystal plane. Since S (the amount of basal plane shear) is approximately 400 times greater than S or S (prismatic shear) it follows that basal plane shear stresses are approximately 400 times less than prismatic plane shear stresses. See Chemistry and Physics of Carbon, Vol. II page 180, Reynolds W. M., Marcel Dekke Inc., New York, l966.

The height or elevation of a particle 4 of FIG. 2 above the surface cannot be determined with a high degree of accuracy because of the extremely small size of the particles and because of the fact that roughness of the surface is greater than the particle size by order of magnitude. In other words, the altitude variations of even a smooth surface (i.e. the elevation differences between the peaks and valleys of the surface undulations) are very substantial as compared with the heights of the particles. By way of example, a relatively smooth surface will have altitude variations of about 10.00001 inches (500 A.) while the height of the particles has been determined to be in the range of about 60 to A. as explained below.

Particle height measurement have been made by a shadow replica technique involving the application of a coating of plastic to a graphitized glass surface and the peeling of theplastic replica from the surface in a manner such that the graphite particles adhere to the plastic. Compacted graphite particles can be removed from a glass surface for the reason that they do not bond, or bond only slightly, to glass. After peeling, a thin film of metallic platinum is deposited from a point source at a known angle to the plastic replica surface and measurements are made of the shadows cast by the adhering graphite particles, that is, regions not covered with platinum. These measurements indicate that the particles have a height of approximately 100 A. However, this FIG. should be taken as an indication of the order of magnitude only and not as a precise measurement. This measurement of the height of the particles does, in any event, indicate that each particle contains a multiplicity of platelets so that further shear of the platelets of the particle will take place when the contact surface is rubbed over a mating contact surface. Specifically, the distance between the basal {0001} crystal plane of a hexagonal graphite crystal plane is 3.40 A., and platelets with a thickness of 100 A. would therefore be capable of undergoing further basal plane shear until they have been divided into a limited number of about 30 atomic platelets. This aspect of the invention is treated below in the theoretical discussion.

The individual particles appear to be securely bonded to the contact surface to the extent that removal of the particle is virtually impossible by any process which does not damage or destroy the surface itself. The tenacity of the bond is demonstrated by the fact that when a surface is prepared in accordance with the invention, the part being treated is cleaned, after deposition of graphite, by immersion in a ultrasonically vibrated bath of Freon TF (trichlorotrifluoroethane). This cleaning step does not remove the bonded particles of graphite of the type shown in the photomicrograph of FIG. 2 and shown schematically in FIGS. 1 and 4. The cleaning does remove excess particles of graphite which are not bonded to the surface and which would not contribute to the lubricating effect of the invention. After such cleaning, about to 20 percent of the surface should remain covered with graphite particles firmly bonded thereto.

The percentage of the contact surface which is covered by graphite particles is not critical. Sufficient graphite should be applied to the surface to achieve the lubrication effect but the surface should not be covered with graphite to the extent that the electrical function of the contact surface will be seriously degraded. If more than 20 percent of the area of the surface is covered by graphite particles, the electrical function of the surface may be degraded to a noticeable extent and if less than 10 percent of the surface is covered, the lubricating effect achieved may be inadequate. For optimum results, about percent of the surface area should be covered with graphite particles.

It has been found that ordinary handling, or even polishing with a buffing wheel, will not remove the graphite from the surface and that any abrasion which does cause removal of the graphite also removes some of the gold plating. It follows from these observations that a surface treated in accordance with the invention does not require special handling and that a surface treatment is durable and long lasting. Graphite is a chemically inert and refractory material so that heat or corrosive atmospheres will not result in degradation of a surface treated in accordance with the invention.

METHOD OF APPLICATION In general, a surface is graphitized in accordance with the invention by forcing a small particle of pure hexagonal graphite against the surface in a manner such that the basal plane of the particle is parallel, or substantially parallel, to the surface after application. The force imposed on the particle at the time of application must be sufficient to cause a stress in the particle which is sufficient to produce basal plane shear in the particle concomitantly with application. Additionally, fresh graphite surface (which results from the shear of the particle) must be brought into contact with the metal surface being graphitized. A particle thus applied to cleaned clean surface will firmly bond to the surface and will, during subsequent movement of the surface, over a complementary surface, undergo further basal plane shear. This further shear lubricates the two surfaces. As will be explained below, when a graphite particle in any one of several orientations is moved against the surface, it will undergo shear if the force of the impact results in a resolved shear stress which is above the shear stress for hexagonal graphite.

A number of commercially available grades of graphite have been used in the practice of the invention and have proved satisfactory. In general, it can be stated that the graphite should be relatively pure (preferably 99 percent more graphite), must be of hexagonal structure, and should have a fine particle size, preferably less than microns. Some commercially available grades which have been used with success are grades 4742, 4735, 5035 supplied by Superior Graphite Company of Chicago, Illinois; grade supplied bySpeerCarbon Products Division, St. Marys, Pa.; and general purpose micro fine flake graphite supplied by A. D. Mackey Inc. of New York, N. Y. All of these grades of graphite have a purity of at least 99 percent, all are hexagonal graphites, and all except Superior 4742 have an average particle size of 30 to 40 microns, Superior 4742 having an average particle size of l micron.

It is, of course, impossible to control the orientation of individual graphite particles being applied to a surface. However, if a sufficiently large number of particles are brought into contact with the surface, some of the particles will be in the orientation required for bonding and these particles will be applied. It follows that in order to achieve a given degree of graphitization of a surface, it is merely necessary to bring a number of particles into engagement with the surface which number is sufficiently large to provide an adequate number of particles having an orientation at the time of engagement which will result in shear and bonding. In the specific processes described below (spraying and buffing) the total number of particles brought into engagement with the surface being treated is probably vastly greater than the number of particles which arrive at the surface in the required basal plane orientation.

The requirement that the particles be stressed at the time of application is satisfied if the particles are sprayed against the surface at a sufficiently high velocity When this method of graphitization is employed, the impact of a particle on the surface gives rise to an internal stress in the particle which leads to shear along the basal plane. When the particles are applied with a buffing wheel, as described below, the pressure of the wheel against the surface produces the shear stress in the particle. A surface could be graphitized by rolling particles against the surface if the rolls and the surface were sufficiently smooth. The stress requirements are not satisfied if the particles are merely sprinkled or lightly brushed on the surface and particles so applied are readily removed by conventional cleaning methods.

The spraying method of graphitizing a surface can be carried out with an apparatus of the type shown in FIG. 3 which comprises a canister 16 mounted on a vibrating unit 17. A plate or disc 18 (FIG. 3A) is mounted inside the canister and a tube 20 extends upwardly through the disc. The disc is provided with a multiplicity of openings having a diameter of about 0.014 inches. A stream of dry nitrogen at a pressure of 50 p.s.i. is introduced through tube 20 and passes downwardly through the openings. This nitrogen stream carries entrained particles through the outlet tube 21 and through the nozzle 23. The stream is directed against the contact surfaces 10 of the terminals and some of these particles adhere to and graphitize the surface in accordance with the invention. The specimen itself may be of any suitable conductive material where a wear problem is anticipated although most of the data have been obtained on gold plated surfaces where the problem has been most acute. The specimen is preferably prepared by thorough cleaning and degreasing prior to the spraying operation.

The amount of time required to deposit the graphite on the surface is quite short. An adequate amount of graphite is obtained on the surface if the specimen is merely passed at the rate of about 56 inches per minute past the nozzle. In order to establish the procedure for a specific process, it is necessary to deposit graphite on some specimens of the surface, examine the specimens and determine if they have the required percent to percent of graphite thereon, and adjust the variables of the process to achieve optimum results. Some specific examples are given below which will provide guide lines for practical operating processes.

After the specimen has been sprayed, it is cleaned in an ultrasonic bath or the equivalent, with a solvent or degreaser to remove any particles which are not bonded to the surface in the manner described above. Where the surface consists of a relatively bright gold plating, its appearance after cleaning is not significantly different from its original appearance. In other words, the graphitized and cleaned surface is bright and shiny and quite often is is impossible, or at least difiicult, to distinguish between a graphitized surface and an untreated surface with the naked eye. Precise light reflectivity measurements will ordinarily reveal the presence of surface treatment in accordance with the invention and, to some extent, can be employed to determine the portion of the surface which is covered with the particles.

It is believed that the bonding of the particles to the surface takes place in the manner schematically shown in FIG. 1. In this Figure it is assumed that a stream of nitrogen is directed against the surface 10 and that particles 4a-4c, having different orientations, are being carried against the surface by the stream. The basal planes of the particles are indicated by the parallel line 6 although it will be realized that any particular particle will actually have a greater number of basal planes than the number shown schematically in FIG. 1. It is also assumed that the particles are moving normally towards the surface 10 and that all of the particles are single crystals. The undulations of the surface I0 (the surface roughness) are ignored in this FIG. in the interest of simplicity.

Some of the particles shown in FIG. I will undergo shear and will be bonded to the surface 10 while others will be reflected. Those particles which bond to the surface must approach the surface in an orientation such that they can undergo shear upon impact. Particle 4a will probably not bond to surface 10 for the reason that its basal planes are orientated parallel to the surface and the impact of this particle, no matter how great, will not result in any shear stresses. Particle 411 will probably not bond although it may undergo shear. The shear of this particle will not, however, result in fresh graphite surface being bought into contact with the surface 10 because of the fact that the basal planes of this particle are oriented normally of the surface 10. Particle 4b could bond, or portions of this particle could bond, if the particle were to fragment at the time of impact in a manner such that fresh graphite surface would be brought into engagement with the surface 10.

Particles 4c, 4d, and 4e will bond to surface 10 if the required shear stress is developed upon impact. It is apparent that when these particles strike the surface, the force of the impact will produce a component parallel to the basal planes 6 of the particles which component, if large enough, will cause shear. When these particles shear, their platelets will be spread over the surface 10 in a manner similar to the manner in which the playing cards of a deck are spread over a surface when the side of the deck is tapped. The fresh graphite surface developed by the shear mechanism comes into contact with, and bonds to the surface 10. A schematic representation of particle 4d after bonding is shown at 4d.

It is quite possible that other crystallographic phenomena might enter into the bonding of the graphite particles to the surface of the contact member, particularly twinning and prismatic slip. These phenomena are ignored in the foregoing discussion for the reason that it is believed that their role is minor as compared to the role of simple shear.

It is toe emphasized that FIG. 1 is intended only as a theoretical model and is presented for purposes of clarification. The precise details of the phenomenon are undoubtedly more complex than FIG. I would imply.

As an alternative to the spraying method of surface graphitization described above, the graphite particles can be ap plied by loading a cloth buffing wheel with small graphite par ticles and pressing the surface against the wheel while it is turning. This method has been successfully practiced and the advantages of increased wear of the surface have been obtained, see example I below.

Referring now to FIG. 5, when two surfaces which have been treated in accordance with the invention are moved relatively over each other, as when two contacts are engaged with each other, the asperities of the surfaces will move across each other and establish electrical contact. It is believed that the lubricating effect of graphite particles in accordance with the invention is achieved by virtue of the fact that the platelets 4 are capable of undergoing further shear after they have been deposited on the surface. As the surfaces move over each other, the graphite platelets which are between the surfaces shear and act to reduce the friction between the surfaces and the galling which otherwise would take place. FIG. 4, like FIG. 1, is intended as a theoretical model which is presented for the purpose of contributing to the understanding of the invention rather than as a pictorial representation.

It will be apparent from the foregoing that the lubricatio phenomenon of the instant invention is not to be confused with the conventional bulk lubricating effect of graphite which is well known to the art. Relatively large amounts of graphite are frequently used for lubricating devices such as locks and bicycle chains, the advantages of graphite over lubricating oils being that graphite is dry and will not attract dust or dirt. This is a bulk lubricating effect however, and the individual particles of graphite are composed of a multiplicity of crystallites (irregular crystals) which are randomly oriented. The particles of graphite used under these circumstances would be completely useless as a lubricant for contact surfaces under low-voltage, low-current conditions since such particles would materially increase the millivolt drop at the contact surface and possibly render the contacts useless for their intended purpose. It has been found, in fact, that if bulk graphite is sprinkled or lightly brushed onto a gold-plated contact surface, the graphite will actually act as an abrasive and accelerate the wear of the plating. Such accelerated wear is probably a result of the fact that the polycrystalline aggregates at the electrical interface will contain single crystallites which have their basal planes oriented normally of the contact surfaces. When such normally oriented crystallites are moved over a contact surface under pressure, they act as minute gouges on the relatively soft and extremely thin gold plating. This submicroscopic gouging effect is not of any significance where graphite is being used as a bulk lubricant in a lock or the like. The lubricating mechanism of the instant invention is clearly a molecular or single crystal phenomenon in which the properties of individual graphite crystallites play the dominant role.

The principles of the invention can be employed in electrical contacts when wear is a significant or potential problem and where it is desired to reduce the force required to engage two contact members with each other. As previously noted, it is contemplated that the invention will find its greatest use in the case of gold contacts because of the high cost of such contacts and the particular problems related thereto. The invention can also be used with contact members plated or composed of silver, nickel and copper or other conductive metals. Where the principles of the invention are used on gold contacts, the advantages of extended wear life, reduced gold plating thickness, reduction in hardness by the use of the gold plating (with attendant improvement in the electrical properties) can all be realized.

While the method of practicing the invention has been described in general terms above, there are presented below some specific examples in which exact techniques are described and the results obtained are presented.

EXAMPLE I A brass disc 1 inch in diameter and 0.013 inches in thickness was electroplated with nickel to a thickness of 50Xl0 inches. It was then electroplated with gold to a thickness of 50X 10 inches, using a cyanide gold bath containing 0.85 percent silver as a brightener for this plating step. The result gold plating has a Knoop hardness of about 110 to 1 15. The disc was thoroughly vapor degreased with trichloroethylene.

One surface of the disc was then graphitized in accordance with the invention by manually pressing the disc for about 5 seconds against a cotton buffing wheel having a diameter of 6 inches which was rotating at a speed of 1,760 rpm. The surface of the wheel had been lightly loaded with graphite filings produced by filing a block of pure graphite.

After buffing the disc was cleaned in an ultrasonic cleaning bath of Freon. The graphitized surface was then examined under a microscope. Particles of the general type shown in the drawing were observed on a minor portion of the surface and the portion so covered was estimated to be about percent of the total contact surface area.

The disc, and a control specimen having an identical plating thereon, were then subjected to a wear test. In this test, each specimen was placed on a reciprocable carriage and a stylus was mounted above the test surface in a fixture in which the stylus was slidably mounted. The styli comprised brass rods one-fourth inches in diameter having spherical bearing surfaces. Each rod was plated with gold over nickel to the same thickness as the test specimen and the control specimen but the surfaces of the rods were not graphitized. A static load of 100 g. was imposed on each rod during reciprocation of the specimens.

After 23 complete back and forth cycles, the surfaces of the specimens were microscopically examined. It was observed that the surface of the control specimen had failed in that the gold plating had been worn off and the nickel was exposed. The coefficient of friction of the control specimen was measured as 0.65 at the beginning of the test and as 1.0 at the end of the 23-cycle life of the specimen.

No significant wear of the test specimen (the graphitized specimen) was observed at the end of 23 cycles. The test was continued and the surface was periodically examined. The gold plating remained intact throughout the test. At the end of 3,550 cycles, the test was stopped and a final examination of the surface was carried out. The gold plating was observed to be intact. The graphite appeared to be streaked along a track when the track was examined at high magnification although the surface still appeared bright and shiny to the unaided eye.

At the beginning of the test, the coefficient of friction of the test specimen was found to be 0.133 and at the end of the test (after 3,550 cycles) it was found to be 0.166.

EXAMPLE ll The control specimen and the test specimen were provided with gold over nickel as platings in example I. The procedure of example I was followed except that a static load of 300 g. was imposed on each stylus during reciprocation of the specimens during the wear test.

The surface of the control specimen was examined after 22 cycles and it was found that the surface had failed in that the gold plating has been worn away in places exposing the nickel undercoat. The coefficient of friction of the control specimen was found to be 0.86, at the beginning of the wear test and 1.0 after 22 cycles.

The test specimen, having a graphitized surface in accordance with the invention, was wear tested for a total of 1,730 cycles at which time the test was stopped. At the end of the test, the gold plating on the test specimen was unbroken and continuous. The coefficient of friction at the beginning of the test was found to be 0.2l6 and at the end of the test was found to be 0. l 84.

EXAMPLE lll Brass discs as described in Example I were provided with platings of 50X 10" inches of nickel and 100x10 inches of gold over the nickel. The gold was deposited from an acid bath containing 0.1 percent cobalt as a hardener. The gold plating has a'Knoop hardness of about 200-240. After plating, the specimens were vapor degreased with trichloroethylene. Portions of the surface of the test specimen were then graphitized by spraying the surface with a stream of nitrogen having entrained graphite particles therein. The spraying apparatus used was a conventional Paasche AUF brush, Model Number 268C supplied by the Paasche Air Brush Company of Chicago, Illinois, the pressure of the nitrogen being l0 p.s.i. When the specimen was sprayed, the nozzle of the air brush was positioned 2 inches from the plated area so that as the test specimen was moved past the brush a band of graphitized surface one-fourth inches in width was produced. During the subsequent wear test, the stylus was positioned against this band as the specimen was reciprocated.

Wear tests as described in example I were then carried out on the control specimen, the static load on the styli being 500 After 25 complete cycles of reciprocation, the surface of the control specimen was examined and it was found that the gold has been worn off of the surface in parts and had been worn off of the bearing surface of the stylus. The coetiicient of friction at the beginning of the test control sample was found to be 0.54 and after 25 cycles it was found to be 0.58.

The wear test of the graphitized test specimen was continued and was stopped after 2,925 cycles at which time the surface of the test specimen was examined. It was found that the gold plating on the surface of the test specimen was unbroken. The coefiicient of friction of the test specimen as the beginning of the wear test was found to be 0.145 and was found to be 0. l 5 at the end of the test.

EXAMPLE lV Two discs as described in example l were electroplated with a plating of 50 10"" inches of nickel and were then electroplated with silver to a thickness of l00 l0"'" inches. The test specimen was graphitized as described in example lll except that the pressure of the nitrogen in the spraying apparatus was 50 p.s.i. and the nozzle was positioned 3 inches from the surface of the test specimen during spraying.

Wear test as previously described were carried out with styli loads of 300 g.

After 75 cycles, the surface of the control sample was found to be worn off in part and the underplating of nickel was visible in places. The coefficient of friction of the control specimen at the beginning of the test was 0.5 and was found to be l.l3 after 75 cycles.

The test specimen was subjected to a total of 3,500 cycles and its surface was found to be intact at the end of the wear test. The coefficient of friction of the test specimen was found to be 0.15 at the beginning of the test and was unchanged from this FIG. at the end of the test.

EXAMPLE V Brass discs as described above were electroplated with 50Xl0 inches of nickel and with 100 l0'"-" inches of gold, the electrolyte in this instance being in Autronex Cl acid gold bath containing 0.1 percent cobalt, this bath being supplied by the Sel-Rex Company and being regarded as a hard gold. The surface of the test specimen was sprayed with a stream of nitrogen having Superior Grade graphite 4,742 entrained therein, the graphite having a particle size of about microns. Spraying was carried out with an S.S. White Air-brasive unit, Model F supplied by the SS White Industrial Division, 201 East 42nd Street, New York, New York.

After treatment of the surface of the test specimen, the two specimens were thoroughly degreased with Freon TF and subjected to a wear test as described above, a load of 300 g. being imposed on the styli during the test. The plating on the control sample was observed to be broken and worn off in places after a total of 400 cycles. The coefficient of friction at the beginning of the test for the control sample was 0.64 and was found to be 0.58 at the end of the wear test.

The test sample was subjected to a total of 2l ,300 wear cycles in the testing apparatus and its surface was found to be unbroken at the end of the test. The coefficient of friction at the beginning of the test for the test specimen was 0.10 and was found to be 0.137 at the end of the test.

In addition to the foregoing specific examples, contact surfaces of a slide switch of the type shown in application Ser. No. 803,223, filed Feb. 28, 1968 by Joseph L. Lockard, have been treated in accordance with the invention with success. Slide switches of the type shown in the Lockard application have nickel contact surfaces on a printed circuit board which are engaged by berylium-copper spring contacts. Under normal circumstances, it has been found that the contact surfaces of these switch devices fail after about 50,000 operating cycles. The berylium-copper spring contacts tend to become completely worn through and the nickel surfaces fail by wear of the plating. Treatment of the nickel surface with graphite in accordance with the invention results in a doubling of the life of the surface.

Changes in the construction will occur to those skilled in the art and various apparently different modifications and embodiments may be made without departing from the scope of the invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only.

We claim:

1. An electrical contact member which is adapted to be engaged with, and disengaged from a complementary member, said member having a metallic contact surface, said surface having discrete single crystal particles of hexagonal graphite thereon, said particles having their basal planes extending substantially parallel to the plane of said surface, said particles being cold welded by molecular bonding to said surface and said particles covering a minor portion of said surface.

2. A contact member as set forth in claim 1 wherein said contact surface is selected from the group consisting of gold,

silver, platinum, palladium, copper, nickel, and alloys thereof.

3. A contact member as set forth in claim 1 wherein the portions of said surface covered by said particles is between 5 per cent and 40 percent.

4. A method of graphitizing the contact surface of an electrical contact device which is subject to sliding engagement with a complementary contact surface, said method comprising the steps of:

cleaning said surface mechanically engaging said surface with particles of hexagonal graphite having a maximum dimension of about microns in a manner to cause stressing of said particles during engagement and shear of said particles with resulting development of virgin particle surface and cold welding of said particles to said contact surface by molecular bonding.

5. The method set forth in claim 4 including the step of cleaning said surface after graphitization thereof.

6. The method set forth in claim 5 wherein engagement of said particles with said surface is effected by spraying said surface with a stream of said particles, the velocity of said particles in said stream being sufficient to cause shear of said particles upon impact.

7. The method set forth in claim 5 wherein the engagement of said particles with said surface is effected by pressing said particles against a moving fabric surface which has been impregnated with said particles.

8. A method of treating an electrical contact surface comprising the steps of:

cleaning said surface,

spraying said surface with a gaseous stream having particles of hexagonal graphite entrained therein, said particles having a maximum dimension in the range of about 40 to 100 microns, said particles having a velocity such that at least some of said particles fracture upon striking said surface and at least some of the resulting fragmentary particles are cold welded by molecular bonding to said surface with their basal planes extending substantially parallel to the plane of said surface, at least some of said particles and particle fragments undergoing shear upon striking said surface, and cleaning said surface after spraying whereby,

Said bonded particles lubricate said surface when said surface is slidably engaged with another contact surface. 

2. A contact member as set forth in claim 1 wherein said contact surface is selected from the group consisting of gold, silver, platinum, palladium, copper, nickel, and alloys thereof.
 3. A contact member as set forth in claim 1 wherein the portions of said surface covered by said particles is between 5 percent and 40 percent.
 4. A method of graphitizing the contact surface of an electrical contact device which is subject to sliding engagement with a complementary contact surface, said method comprising the steps of: cleaning said surface mechanically engaging said surface with particles of hexagonal graphite having a maximum dimension of about 100 microns in a manner to cause stressing of said particles during engagement and shear of said particles with resulting development of virgin particle surface and cold welding of said particles to said contact surface by molecular bonding.
 5. The method set forth in claim 4 including the step of cleaning said surface after graphitization thereof.
 6. The method set fortH in claim 5 wherein engagement of said particles with said surface is effected by spraying said surface with a stream of said particles, the velocity of said particles in said stream being sufficient to cause shear of said particles upon impact.
 7. The method set forth in claim 5 wherein the engagement of said particles with said surface is effected by pressing said particles against a moving fabric surface which has been impregnated with said particles.
 8. A method of treating an electrical contact surface comprising the steps of: cleaning said surface, spraying said surface with a gaseous stream having particles of hexagonal graphite entrained therein, said particles having a maximum dimension in the range of about 40 to 100 microns, said particles having a velocity such that at least some of said particles fracture upon striking said surface and at least some of the resulting fragmentary particles are cold welded by molecular bonding to said surface with their basal planes extending substantially parallel to the plane of said surface, at least some of said particles and particle fragments undergoing shear upon striking said surface, and cleaning said surface after spraying whereby, said bonded particles lubricate said surface when said surface is slidably engaged with another contact surface. 